1. Field of the Invention
The present invention relates to a method of fabricating an image sensor, and more particularly, to a method of fabricating a complementary metal-oxide semiconductor (CMOS) image sensor.
2. Description of the Prior Art
Charge-coupled devices (CCDs) have been the mainstay of conventional imaging circuits for converting light into electrical signals. The applications of CCDs include monitors, transcription machines, and cameras. Although CCDs have many advantages, CCDs also suffer from high costs and the limitations imposed by their size. To overcome the weaknesses of CCDs and to reduce costs and dimensions, CMOS photodiode devices have been developed. Since a CMOS photodiode device can be produced using conventional techniques, both the cost and the size of the sensor can be reduced.
Whether an image sensor device is composed of CCD or CMOS photodiode, incident light striking the image sensor must be separated into combinations of light of different wavelengths in order to properly sense color images. The intensities of the different wavelengths of the light is received by sensor devices and is transformed into electrical signals which are used to determine the original color of the incident light. To accomplish this feat, a color filter array (CFA) must be formed on each photosensor device.
FIG. 1 through FIG. 3 are schematic diagrams in a conventional method of fabricating a CMOS image sensor 38 on a semiconductor substrate 10. The CMOS image sensor 38 comprises a P-well 12and a sensor array region I positioned on the P-well 12. The sensor array region I comprises a plurality of photodiodes (not shown) positioned on the P-well 12 and a plurality of shallow trench isolations (STI) 14 formed in the P-well 12 surrounding the photodiode. Each photodiode comprises a CMOS transistor (not shown) electrically connected to a photosensor area 16. The STI 14 acts as a dielectric insulating material to prevent short-circuiting due to contact between the photosensor area 16 and other components.
First, a planarizing layer 18 is coated on the semiconductor substrate 10 and covers each photosensor area 16. Then, a plurality of metal barriers 20 is formed on the planarizing layer 18 in the sensor array region I of the semiconductor substrate 10. The metal barriers 20 are located above each STI 14 and are used to prevent scattering of incident light 39. A patterned metal layer is formed on the semiconductor substrate 10 outside the sensor array region I and is used as a bonding pad metal layer 22. Afterwards, a planarizing layer 24 is coated on the semiconductor substrate 10. Then a patterned photoresist layer (not shown) is formed on the planarizing layer 24 outside the sensor array region I to define a pattern of a bonding pad opening 26. Thereafter, an etching process is performed to form the bonding pad opening 26 down to the bonding pad metal layer 22.
A red color filter layer 28 is formed on the sensor array region I of the semiconductor substrate 10. The color filter layer is composed of a positive type photoresist containing a red dye in an amount of 10 to 50 wt % (dry weight). Then, a photo-etching process (PEP) is performed to form a red color filter array (CFA) 28 in the red color filter layer corresponding to the respective photodiode. To increase the effectiveness and reliability of the red CFA 28, an ultra-violet (UV) light-irradiation process and a heating process can be performed after the formation of the red CFA 28. The UV light used has a wavelength of about 320 nm or less, and a quantity of about 20 J/cm2 or less. The heating process is best performed in an inert atmosphere, for example in nitrogen (N2), to suppress the oxidation of the photoresist material. The starting temperature of the heating process is between 60 and 140xc2x0 C. Then, an average increasing temperature rate used in the heating process is 1.5xc2x0 C./sec. The end temperature of the heating process is between 160 and 220xc2x0 C. A green CFA 30 and a blue CFA 32 are formed by repeating the above-mentioned processes with dyes of different colors. Thus, an R/G/B CFA comprises a red CFA 28, a green CFA 30 and a blue CFA 32.
A spacer layer 34 is formed on the R/G/B CFA, and a polymer layer (not shown) composed of acrylate material is formed on the spacer layer 34. Further, an exposure, development, and annealing process is performed to form a plurality of U-lenses 36 in the polymer layer corresponding to the respective R/G/B CFA, and fabrication of the CMOS image sensor 38 is completed.
When incident light 39 entering the CMOS image sensor 38 is focused by the U-lens 36, it passes through the R/G/B CFA, which only transmits light of a specific wavelength. The incident light 39 is transferred to the corresponding photosensor area 16, which transforms the incident light 39 into electrical signals so as to obtain the original color of the incident light 39.
In a conventional CMOS image sensor 38, the U-lens 36, the spacer layer 34 and the R/G/B CFA are all made of photoresist materials with low-temperature flash points of around 300xc2x0 C. or less. Therefore, a passivation layer cannot be formed on the U-lens 36 to protect the U-lens 36 and the R/G/B CFA from loose particles or other contamination sources. Other drawbacks of the conventional CMOS image sensor 38 are listed below:(1) Because the U-lens 36 and the R/G/B CFA are made of photoresist materials, the bonding pad has to be formed prior to the formation of the R/G/B CFA and the U-lenses 36. However, the R/G/B CFA development process will attack the bonding pad metal layer 22 and creates a risk of pitting. (2) Since the bonding pad is formed prior to the R/G/B CFA steps, a large trench on a scribe line will induce some color wave images on the CFA. (3) Dropped particles cannot be removed using a jet clean process because there is no passivation layer on the U-lens 36. This means that contamination during the manufacturing process requires that the whole U-lens 36 and the R/G/B CFA be removed and recreated. (4) The conventional technique utilizes the high curvature U-lens 36 to adjust the focal plane of the incident light 39 passing through and focused by the U-lens 36. As the process size in semiconductor manufacturing decreases, formation of the high curvature U-lens 36 becomes increasingly difficult. (5) A space exists between each U-lens 36, so that scattered light easily enters the neighboring photosensor area 16, resulting in cross talk effects. This increases the noise levels of the CMOS transistor image sensor 38 and reduces sensitivity.
It is therefore an objective of the claimed invention to provide a method of fabricating an image sensor having a passivation layer.
It is another objective of the claimed invention to provide a method of fabricating an image sensor in which the focal plane of incident light can be adjusted.
The claimed invention involves providing a semiconductor substrate comprising a sensor array region. First, a planarizing layer is formed on the semiconductor substrate. An R/G/B color filter array (CFA) is formed on portions of the planarizing layer corresponding to the sensor array region, and a spacer layer is formed on the R/G/B CFA. A plurality of U-lenses is formed on the spacer layer corresponding to the R/G/B CFA, with a space between each U-lens. Finally, a buffer layer is applied to fill the space between the U-lens, and a low-temperature passivation layer is deposited on the buffer layer and the U-lens.
The present invention utilizes the buffer layer and the low-temperature passivation layer sequentially formed on the U-lens to prevent damage to the U-lens. Because the buffer layer has a predetermined index of refraction (IR), an optical path of incident light can be changed by adjusting the IR of the buffer layer. Simultaneously, cross talk effects, caused when incident light is refracted in a way that causes it to strike an adjacent photosensor, can be prevented, and quantum efficiency (QE) can be increased. The low-temperature passivation layer is formed at a temperature of about 300xc2x0 C. or less. Because this is below the flash point of the U-lens and the CFA, they are not are not effected by this process. The passivation layer prevents the contamination of the U-lens by particles or other sources, and increases product reliability.